Hi maybe to late but hope this helps , I agree they ran small, for me 1 size but I noticed the T bar that can cause the ankle straps to become short and limit the amount of holes available if you have very high in steps.
I wear a 37 on average yet needed a 38 but I have weird shaped feet , good luck Sharon
My Ditassima's are the same size as my Pigalle Follies and So Kate, and fit perfectly. I wear a 8.5 US and a 39 is perfect. You could size up as the strap helps keep your foot in, but I wouldn't size down.